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Unformatted text preview: 1732 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, NO. 8, AUGUST 2007 A Transient-Enhanced Low-Quiescent Current Low-Dropout Regulator With Buffer Impedance Attenuation Mohammad Al-Shyoukh, Hoi Lee , Member, IEEE , and Raul Perez , Member, IEEE Abstract This paper presents a low-dropout regulator (LDO) for portable applications with an impedance-attenuated buffer for driving the pass device. Dynamically-biased shunt feedback is pro- posed in the buffer to lower its output resistance such that the pole at the gate of the pass device is pushed to high frequencies without dissipating large quiescent current. By employing the cur- rent-buffer compensation, only a single pole is realized within the regulation loop unity-gain bandwidth and over 65 phase margin is achieved under the full range of the load current in the LDO. The LDO thus achieves stability without using any low-frequency zero. The maximum output-voltage variation can be minimized during load transients even if a small output capacitor is used. The LDO with the proposed impedance-attenuated buffer has been implemented in a 0.35- m twin-well CMOS process. The proposed LDO dissipates 20- A quiescent current at no-load condition and is able to deliver up to 200-mA load current. With a 1- F output capacitor, the maximum transient output-voltage variation is within 3% of the output voltage with load step changes of 200 mA/100 ns. Index Terms Linear regulator, load transient response, low- dropout regulator (LDO), pass device, power management inte- grated circuits, voltage buffer. I. INTRODUCTION P OWER management is essential in all battery-powered portable devices such as cellular phones and PDAs in order to reduce the standby power and prolong the battery runtime. Low-dropout regulators (LDOs) are one of the most critical power management modules, as they can provide regulated low-noise and precision supply voltages for noise-sensitive analog blocks. With the widespread proliferation of modern portable devices, ever more stringent performance requirements of the LDO are needed. First, low dropout voltage across the pass device of the LDO is required provide high power effi- ciency. In addition, the increased level of integration in portable devices not only demands the LDO to deliver high load current, but also requires the no-load quiescent current of the LDO to be minimized for improving the current efficiency [1]. Good load transient response with small output-voltage variation including overshoots and undershoots upon load switching is critical to Manuscript received November 10, 2006; revised April 12, 2007. This work was supported by Texas Instruments Inc. M. Al-Shyoukh is with Texas Instruments Inc., Dallas, TX 75243 USA, and also with the Department of Electrical Engineering, University of Texas at Dallas, Richardson, TX 75083-0688 USA (e-mail: mshyoukh@ti.com)....
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